We have studied the adsorption of lysozyme and pentaethylene glycol monododecyl ether (C₁₂E₅) at the air/water interface using neutron reflection and surface tension measurements. The effect of C₁₂E₅ concentration was examined at three fixed lysozyme concentrations of 0.01, 1 and 4 g dm⁻³. The surface tension showed little variation with the addition of C₁₂E₅ over the low surfactant concentration region, but with the increase of C₁₂E₅ concentration, the surface tension gradually became identical to that corresponding to pure C₁₂E₅. These results suggest a progressive replacement of lysozyme by C₁₂E₅ and that the observed surface event is dominated by competitive adsorption. The parallel neutron measurements showed that, at low surfactant concentration, the surface was predominantly occupied by lysozyme. At intermediate C₁₂E₅ concentrations, the surface layer consisted of both lysozyme and C₁₂E₅, with the C₁₂E₅ eventually completely replacing the adsorbed lysozyme as the surfactant concentration was further increased. While the neutron results confirm the inference from surface tension measurement, structural analysis clearly showed the partial breakdown of the globular structure of lysozyme induced by the nonionic surfactant. Furthermore, neutron data showed that the adsorbed C₁₂E₅ molecules are present at the top surface layer only, suggesting no preferential association or binding between the surfactant and any immersed protein fragments at the interface.